Particle emissions can have an adverse effect on the environment, having the potential, depending on level of exposure, composition, and size-distribution, to be both profoundly harmful to human health and detrimental to ecosystems and man-made infrastructure alike. As a result, many industries face ever increasing pressure to monitor, reduce and/or limit certain emissions generated by internal combustion engines, stacks, other systems that generate emissions, or other sources.
Vehicle and transportation sector-related emissions continue to be a leading source of greenhouse gas (GHG) and air pollution in urban areas around the globe. As an example, there were over 260 million vehicles in the United States (U.S.) in 2012 that emitted 33% (1,750 million metric tons) of total U.S. CO2 emissions. In the same year, the U.S. transportation sector share of total U.S. emissions for CO, NOx, and particulate matter (PM) were 54%, 59%, and 8%, respectively. Therefore, significant resources continue to be focused on emission reduction tactics which typically fall into two categories: current fleet inventory upgrade (e.g., roadside and/or engine bay inspection and maintenance (I/M) programs, aftermarket engine/vehicle/fuel programs, etc.) or new vehicle manufacturing (e.g., revisions of standards for newly manufactured vehicles, etc.).
When fine particulate was first identified as a possible health hazard, environmental agencies found it most convenient and accurate to measure particulate emissions in terms of how much particulate mass was emitted from a given pollution source or how much mass was contained in the ambient air we breathe. However, as particulate sources became cleaner in response to regulations and consumer demand, investigators discovered that the number of ultra-fine particles in ambient air is more closely correlated with health effects than the total mass of those particles.
The U.S. Environmental Protection Agency (USEPA) defines PM as a complex mixture of extremely small particles and liquid droplets made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles. The USEPA has employed the concept of vehicle exhaust PM evaluation since the 1970's starting with the capture of particulate emission samples on filter paper collected from samples of vehicle exhaust tested in laboratories. The mass collected samples were subsequently determined using a gravimetric process. The PM was then properly catalogued and documented.
More recently, the development of a particle number (PN) measurement system by the European Union in 2007 enabled a more accurate and repeatable measurement of the number of particulates emitted. Additional EU objectives were to minimize required changes to the current type approval facilities, to employ an understandable metric, and for the system to be simple to operate.
Accurate emission(s) data are needed in order to properly evaluate the impact of emission reduction strategies. In doing so, it may be important to differentiate between the differing sizes of PM/PN in order to better understand both the process that produced them and the potential solution(s) for their reduction. Specifically, atmospheric particulate sizes typically range from a few nanometers up to several micrometers. Coarse particles larger than one micrometer are dominated by biological sources (e.g., spores, pollen, bacteria, etc.) or mineral sources.
Fine particles (less than a micrometer) are typically elemental carbon or leftover constructs from gases such as sulfates, nitrates, or organic carbon. It is the PM/PN that originates from the combustion processes which tend to be of significant interest. The size range of elemental carbon (e.g. “ultra-fines”), or particles smaller than 0.1 micrometer, is dominated by such particles due to the combustion process associated with on-road, off-road, and non-road transportation activities. Such particulates have been cited as dangerous due to toxic trace compounds (e.g., heavy metals, polycyclic aromatic hydrocarbons, etc.). The USEPA and the European Union's Joint Research Centre (JRC) have declared that the concentration of such particles is highly variable, and appears to demonstrate a significant pattern of variation, especially close to urban areas and traffic congestion.
In addition, the early focus of regulation and, therefore, the manufacturing sectors was on PM which led to a change in the particle size distributions produced by many regulated particulate sources. For example, earlier generation vehicles tended to emit larger masses of particulate dominated by very coarse (and often visible) matter, while newer vehicles tend to emit less by mass but much smaller (and arguably, based on current scientific understanding, more harmful) particles. As these global standards continue to become stricter and the nature of the particles that they were introduced to manage evolves, it is imperative that both measurement technologies and associated evaluation systems be developed that maximize our potential to implement effective emission abatement strategies.
One challenge currently faced is the routine calibration and bench-marking of measurement methods that quantify finer particulate emissions and/or coarse-to-fine size distributions. As an example, opacity based monitoring technologies have been widely used in vehicle testing programs, such as in the California Heavy Duty Vehicle Inspection Program (HDVIP). Instruments used in these programs can also be reliably calibrated using “optical transfer reduction,” which involves, for example, using a glass lens with an accurately measured degree of surface etching placed in the optical analyzers light beam path. This means both the test method and the validation procedure can be performed at roadside locations allowing “per-vehicle” management of vehicle particulate emissions. However, this system is only amenable to the regulation of larger/coarser particulates and is not sensitive to the finer PM emitted from more modern trucks.
There are a range of monitoring technologies that allow both the characterization of particulate size distribution and measurement of the finer particulate emissions that now make up a component of particulate emissions. There are also multiple methods for the calibration and testing of such systems, but these typically rely on the use of laboratory-specified and designed equipment. This reliance is, in part, because the standards have to be extremely exacting in order to faithfully replicate actual “emission events” and in part because alternative commercially-scalable options have not yet been identified. As an example, recent PM/PN calibration systems and particulate generators utilized in the automotive industry have attempted to faithfully recreate soot, nano-particles, and carbon pollutants. Not only is this approach exceedingly costly, it also requires a significant amount of power, heat, and reinforced housing to contain the process required for the generation of such particulates, restricting its application anywhere except highly controlled laboratory environments.
What are needed are particulate generation/calibration approaches that can be applied on a more routine basis, such as for the routine bench-marking and quality assurance of PM, PN, or particulate size distribution data collected in real-world portable emission measurement system (PEMS) studies, large scale dynamometer testing procedures, and future “per-vehicle” testing approaches similar the existing HDVIP scheme.